This invention relates to a system for controlling the torque of a polyphase induction motor.
In a position control system and a speed control system where quick response is necessary, shunt DC motors have generally been used. Because the output torque of the shunt motor is theoretically proportional to the armature current so that the control system can be constructed according to a theory of linear automatic control and it is possible to obtain a control system having a quick response characteristic contemplated by the designer.
If it is possible to use an induction motor in such control system as a constant speed motor, there are the following advantages over a DC motor.
More particularly, as there is no commutator, it is easy to maintain and the motor is rugged and free from spark and electric noise. Moreover, as there is no problem of commutation, it is possible to operate the motor at a high current and a high speed. Furthermore, induction motors have excellent dust proofness and explosion proofness and can be manufactured to have a small size at low cost.
Among prior art control systems of an induction motor may be mentioned a so-called V/f constant control system in which the voltage V and the primary frequency f are controlled to be proportional to the speed of the motor. According to this system it is impossible to directly determine the voltage V and the frequency f that can produce a torque having a value instructed by the control system whereby it has been impossible to generate the torque according to the theory of linear automatic control.